A known hybrid vehicle has an engine, a motor/generator) used for driving (or propelling) the vehicle, for starting the engine and for power generation and a clutch that is disposed between the engine and the motor/generator. When such a hybrid vehicle is performing regenerative braking (regenerative deceleration) by means of the motor/generator, a loss of electric energy regeneration, which may occur due to engine friction, can be minimized by disengaging the clutch to disconnect the engine from a driving power transmission and the motor/generator, thereby ensuring an amount of electric energy regeneration at the maximum efficiency.
Such a hybrid vehicle, which includes a clutch between an engine and an electric motor/generator, has been disclosed in Japanese Patent Provisional Publication Nos. 2004-162534 and 2006-306328. For vehicle propulsion, such hybrid vehicles switch between operation in an EV mode (electric drive), in which only the electric motor/generator is used as the propelling power source, and an HEV mode (hybrid drive), in which at least the engine is used as the propelling power source.
In setting an operating range (an electric-drive operating range) of the hybrid vehicle, a hysteresis for mode-switching between the EV and HEV modes is provided to avoid repeated starting and stopping of the engine resulting from switching between the EV and HEV modes in response to frequent driver accelerator operations. For instance, assuming that the electric-drive operating range (the EV operating range) is defined by two parameters, namely, vehicle speed and accelerator opening, a threshold value for a transition from the EV mode to the HEV mode is set as an engine-start line and a threshold value for a transition from the HEV mode to the EV mode is set as an engine-stop line.